Traditional filter bags (or tea bags), are generally formed from one or more lengths of filter material which are suitably folded and joined to make one or more chambers, each containing a dose of product intended for infusion in liquid (water).
The filter bag thus formed may be combined with a string which is joined at one end to the filter bag and at the other end to a tag, used to handle the filter bag during infusion.
This “traditional” type of filter bag, with its flat configuration, is extremely practical because it is relatively easy to make in current machines and also quick and easy to package.
On the contrary, their shape (substantially flat) makes filter bags of this kind unsuitable for containing infusion products of different, coarse-sized kinds, such as granules and leaves, which need a different containment chamber, that is to say, a larger chamber with more space for infusion to allow the passage of a larger amount of liquid and thus to obtain a better quality infusion.
For this reason, tea bags or filter bags having a three-dimensional shape, in particular a tetrahedral shape, have been devised and made.
The three-dimensional configuration makes these filter bags not only attractive but also functional because they are easy to handle for infusion and have a large chamber where the product can come into contact with the liquid.
These advantages of the three-dimensional configuration of the filter bag are counterbalanced by some disadvantages, however.
One disadvantage is due to the complexity of making a three-dimensional filter bag from a traditional strip of filter material (visible also in patent document EP 1.549.548 in the name of the same Applicant as this invention).
In light of this, the three-dimensional filter bag is made by a machine which, essentially, feeds a continuous strip of filter material.
A first sealing station closes the continuous strip onto itself and makes a first longitudinal seal to form a continuous tubular strip.
Next, a second station makes a sequence of second seals, transversal to the first seal, on the tubular strip to define successive flat filter bag perimeters.
After that, a tubular strip cutting station cuts the tubular strip at the transversal seals to obtain a sequence of partly formed filter bags, each having a closed (sealed) bottom end and an open, unsealed top end.
The machine comprises an opening station for opening out the unsealed end of each filter bag and an infusion product dosing station which places a dose of infusion product into the open mouth of the filter bag.
A further third sealing station makes a third seal by which the still open top end of each filter bag is closed.
It should be noted that the third closing seal extends transversely to both the first longitudinal seal and to the second transversal (ST) in order to obtain a three-dimensional, tetrahedral shape.
As may be noted, therefore, the process of forming the filter bag requires three seals, even on top of one another, to form, in a plane, a sort of H shape lying on edge, but angularly offset from each other in space.
As a result, the filter bag may not be properly sealed because the filter material may be damaged by the sequence of thermal shocks caused by the different sealing steps.
The step of opening out and partly rotating the top end of the filter bag before definitively closing it, besides being extremely critical and strongly dependent on the rigidity of the filter material, causes elasto-plastic torsion of the filter material, with the risk of damaging or crushing the product contained therein (granules or leaves), thereby reducing the end quality of the product.
The large number of steps required to form filter bags of this shape also necessitates, as may be inferred from the above, a large number of stations which, in practice, means that the machines used to make them are complex and expensive.
Moreover, the large number of stations also makes the productivity of the machines per unit time relatively low, especially if compared to those used to make traditional filter bags.
The different stations used for sealing at different times (and requiring the filter bag to be manipulated in different ways) also means that the sealing operations must be extremely precise.
In light of this, the filter bags are often imperfect in appearance.
Another type of machine for making tetrahedral filter bags is known from international patent application WO 95/01907, which describes a machine having what is known as a vertical-axis tie around which a strip of filter paper is wound to form a closed tube, a doser by which a dose of product is dosed into the closed tube and two wheel-like sealing elements positioned at 90° to each other about a vertical axis. These sealing elements make seals on the closed tube of filter paper which are in turn at 90° to each other about the vertical axis of the tie to form a filter bag of tetrahedral shape.